Hyperoxia-induced converting enzyme insufficiency in conscious rat: cardiovascular effects

Hyperoxia-induced arterial compliance decrease in healthy man

INTRODUCTION

Oxygen therapy is commonly used in emergency department and intensive care units without careful evaluation of its effects, especially on the haemodynamics and artery characteristics.

MATERIALS AND METHODS

A prospective laboratory study evaluated brachial circulatory effects of normobaric hyperoxia using ultrasonography-Doppler. The study was set in a hospital research laboratory. The subjects were thirteen healthy volunteers. Investigations were performed under normal air ventilation and after 20 min of hyperoxic mixture ventilation using a high concentration mask. Two dimensional images and brachial blood flow velocities were recorded using ultrasonography and pulsed Doppler to study changes in cross sectional area, blood flow, resistance index, and cross-sectional compliance coefficient.

RESULTS

During hyperoxic exposure, mean PaO2 was 372 +/- 21 mmHg. A significant decrease of heart rate was observed. Arterial pressures (systolic and diastolic arterial pressures) were not modified. A decrease of cross sectional areas at end diastole and end systole was observed. Pulsed Doppler study showed a decrease of brachial artery blood flow and an increase of the resistance index. Furthermore, a decrease of the cross-sectional compliance coefficient was observed during hyperoxic exposure in all subjects.

CONCLUSION

This study using two-dimensional ultrasonography and pulsed Doppler could demonstrate an increase in brachial arterial tone and a decrease in brachial blood flow under normobaric hyperoxia.

Hyperoxia increases salt intake in spontaneously hypertensive rats (SHR)

The effect of normobaric hyperoxia on voluntary salt intake was investigated in 20 sham-operated and 11 carotid body-denervated SHR. While in sham-operated SHR the saline intake was enhanced during the whole hyperoxic period, the carotid body-denervated rats showed an increase in salt appetite only on the second and third day of hyperoxia. Water intake was not significantly different in sham-operated and carotid body-denervated SHR. These findings, together with our previous results, suggest that chemoreceptor activity determines salt appetite but not water intake in SHR.

Renin-angiotensin II response to the hemodynamic pathology of ovines with ventricular septal defect

We studied the response of the renin-angiotensin system (RAS) to a surgically created ventricular septal defect (VSD) in immature ovines and also the role of angiotensin II in the pathophysiology of VSD in the chronically instrumented ovine. Plasma renin activity (PRA) was increased from 2.39 +/- 1.1 to 3.78 +/- 1.4 ng/ml/hr (p less than 0.05, n = 17) after VSD but not after sham procedure. The change in PRA was positively correlated with the amount of left-to-right shunt through the VSD (r = 0.74, p less than 0.05). Inhibition of angiotensin II effect with saralasin (10 micrograms/kg/min) or angiotensin II production with captopril (2 mg/kg) lowered systemic resistance (Rs) by 14% and 34%, respectively (p less than 0.05), and raised pulmonary resistance (Rp) by 35% and 77%, respectively (p less than 0.05). Thirty minutes following captopril, the ratio of pulmonary to systemic flow (Qp/Qs) decreased from 3.31 +/- 0.18 to 2.15 +/- 0.18 (p less than 0.05) while total pulmonary flow fell from 7.15 +/- 0.38 to 5.92 +/- 0.34 l/min/M2 (p less than 0.05, n = 11). Systemic flow increased from 2.17 +/- 0.14 to 2.86 +/- 0.33 l/min/M2 (p less than 0.05) despite a reduction in left atrial pressure (17.3 +/- 1.0 vs. 13.0 +/- 1.7, p less than 0.01). Reinfusion of angiotensin II (0.02 micrograms/kg/min) into the central aorta after captopril returned the hemodynamics to baseline including a rise in Rs and fall in Rp. Exogenous angiotensin II alone (0.08 micrograms/kg/min) or a threefold stimulation in PRA with furosemide (2 mg/kg) caused little hemodynamic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Lung MK 351A uptake after hypoxia adaptation and subsequent hyperoxia exposure

This study investigated the effects of hypoxia adaptation (10% O2 for 4 days) on rat lung angiotensin-converting enzyme (ACE) content before and after hyperoxia exposure (greater than 95% O2 for 2 days). The rationale for this investigation was that hyperoxia exposure decreases lung ACE, while hypoxia adaptation produces tolerance (improved survival) to oxygen toxicity in rats. Rats were exposed to air, hypoxia, hyperoxia alone, or hypoxia followed immediately by hyperoxia. The lungs were then excised and perfused in vitro at 12 ml/min with buffer. Lung ACE content was quantitated by measuring the single-pass binding of an iodinated ACE inhibitor, 125I-MK 351A, a derivative of lisinopril. We showed previously that 125I-MK 351A binding correlates quantitatively with ACE activity in lung homogenates and isolated, perfused lungs. Lung internal surface area was estimated by measuring the mean alveolar diameter of 5 micron hematoxylin and eosin sections from lungs fixed in inflation (25 cmH2O transpulmonary pressure). Hypoxia adaptation per se had no effect on 125I-MK 351A binding or estimated alveolar surface area, while hyperoxia exposure caused a significant decrease in both 125I-MK 351A binding and alveolar surface area. These hyperoxia-induced decreases were prevented partially by hypoxia adaptation, indicating a protective effect on both ACE content and surface area. 125I-MK 351A binding in isolated perfused lungs changed in parallel with histologically estimated surface area. These results indicate that hypoxia preadaptation minimizes the oxygen-induced decrease in lung microvascular ACE content.

Interaction between oxygen and cell membranes: modification of membrane lipids to enhance pulmonary artery endothelial cell tolerance to hypoxia

Because high partial pressures of oxygen (O2) can cause peroxidative cleavage of membrane lipids, it is plausible to hypothesize that hyperoxia alters the physical state and composition of lipids in the membranes of pulmonary endothelial cells and that manipulation of the lipid profile may modify endothelial cell tolerance to hyperoxic injury. To test this, porcine pulmonary artery endothelial cells were exposed to 95% O2 at 1 atmosphere absolute (ATA) in the presence or absence of cis vaccenic acid (CVA), a monounsaturated fatty acid (C18:1#11). Plasma membrane fluidity was assessed by fluorescence spectroscopy, plasma membrane lipid composition was quantitated using thin layer and gas chromatography, and cytotoxicity was monitored by measuring release of lactate dehydrogenase (LDH). Hyperoxia caused peroxidation of membrane lipids and decreased fluidity in three distinct lipid domains within the plasma membrane. Incubation with CVA was associated with a reduction in the degree of unsaturation of the constituent fatty acids within all plasma membrane lipid subclasses except monoglycerides. CVA-treated cells were also more resistant to hyperoxic injury as judged by LDH release. These results support the hypothesis that cells with membranes in which the fatty acyl chains are more resistant to the disordering effects of high O2 tensions may be more resistant to O2 toxicity.

Hypoxia-induced oxygen tolerance: maintenance of endothelial metabolic function

Hypoxia (10%-12% O2) preadaptation for 4-7 days effectively protects rats from oxygen toxicity. The present study was designed to investigate the hypothesis that the lung's microvascular endothelium shares in development of oxygen tolerance and therefore that endothelial metabolic function would be protected from oxygen toxicity by prior adaptation to hypoxia. Since pulmonary oxygen toxicity decreases lung capillary angiotensin converting enzyme (ACE) activity, we assayed converting enzyme active sites in an isolated perfused rat lung preparation as a marker for the development of oxygen toxicity and tolerance. Rats were exposed to air, hypoxia (10% O2 for 4 days), hyperoxia (greater than 95% O2 for 2 days) alone, or hypoxia followed immediately by hyperoxia. Lung vascular ACE content was quantitated by measuring the single pass binding of an iodinated-converting enzyme inhibitor, 125I-MK351A, a derivative of lisinopril. Hypoxia adaptation per se had no effect on ACE content reflected in normal 125I-MK351A binding, whereas hyperoxia exposure caused a significant decrease in lung vascular ACE. Hyperoxia-induced decreases in ACE content were prevented partially by hypoxia adaptation, indicating that ACE content on luminal endothelial surfaces was protected from oxygen toxicity. In isolated perfused lungs 125I-MK351A binding reflects development of oxygen tolerance after hypoxia preadaptation and suggests that lung endothelial metabolic function is protected from oxygen toxicity.